1. Field of the Invention
This invention relates to new water soluble or dispersible polyurethanes containing carboxylate groups and ammonium groups as counterions to the carboxylate groups, a process for the preparation of such polyurethanes and the use of these polyurethanes in the form of their aqueous solutions or dispersions as coating compounds for substrates.
2. Description of the Prior Art
Aqueous solutions and dispersions of polyurethanes are known. DE-AS No. 1,495,745, for example, describes inter alia aqueous dispersions of polyurethanes containing carboxylate groups in which in particular ternary ammonium ions based on volatile tertiary amines are used as counterions for the carboxylate groups.
U.S. Pat. No. 3,412,054 describes dispersions of polyurethanes containing carboxylate groups which have only a limited capacity for dilution with water and require the presence of a cosolvent. The number of built-in carboxylate groups required for enabling the polyurethanes to be diluted with water lies in the range of 30 to 112 milliequivalents per 100 g. In the examples given, dimethylethanolamine is the only neutralizing agent used.
Compared with polyurethanes which have been rendered hydrophilic purely by means of ions, polyurethanes which have non-ionic, hydrophilic polyethylene oxide segments built into the polymer structure form dispersions which have improved resistance to water, electrolytes and frost but their thermostability is reduced (DE-OS Nos. 2,314,512, 2,314,513). For the quality level of aqueous polyurethane urea dispersions it has been found particularly advantageous to use a combination of chemically incorporated hydrophilic polyethylene oxide segments with chemically incorporated ionic hydrophilic centers (DE-OS Nos. 2,551,094, 2,651,506, EP-A-000,347). The products obtained were found to have advantages with respect to flow properties, resistance to coagulation by electrolytes and low temperature and resistance to water and solvents.
Apart from Example 11 of DE-AS No. 1,495,745 describing the preparation of dispersions containing ammonium groups as counterions, which are found to be unsatisfactory for practical application, the prior art teaching discloses in particular that for the preparation of aqueous dispersions or solutions of polyurethanes containing carboxylate groups the agents used for neutralizing the carboxylate groups are preferably tertiary amines since the use of a neutralizing agent containing isocyanate reactive groups leads to undesirable side reactions. Thus, for example, in U.S. Pat. No. 4,408,008 (column 9, line 46 to column 10, line 1) it is explicitly recommended to use, as neutralizing agents, amines which do not contain any isocyanate reactive hydrogen atoms.
The advantage of using readily volatile tertiary amines in the preparation of compounds which are rendered hydrophilic by means of ionic groups is due to the fact that the amines are removed by diffusion when the polyurethane dispersions produced by such a process are applied as coatings, with the result that the water-resistance of the coatings is enhanced. The disadvantage of such readily volatile tertiary amines, however, lies in their ecological and toxicological harmfulness.
It was therefore an object of the present invention to provide water-soluble or dispersible polyurethanes containing carboxylate groups in which the following advantages would be combined:
(1) the counterions for the carboxylate groups should be readily volatile so that coatings prepared from the aqueous solutions or dispersions of polyurethanes would be highly water-resistant; PA1 (2) the base of the counterions should be physiologically substantially harmless compared to readily volatile organic amines; and PA1 (3) the aqueous solutions or dispersions should be stable in storage but at the same time provide to coatings which are equal in quality to those obtained from known dispersions, i.e. would have superior properties when compared to the coatings obtained from the dispersions according to Example 11 of DE-AS No. 1,495,745. PA1 (d1) diisocyanates and/or compounds which are difunctional in isocyanate polyaddition reactions and contain isocyanate reactive hydrogen atoms, which have polyether lateral chains containing ethylene oxide units, PA1 (d2) monoisocyanates and/or compounds which are monofunctional in isocyanate polyaddition reactions and contain at least one isocyanate reactive hydrogen atom, which have polyether chains containing ethylene oxide units, and PA1 (d3) mixtures of compounds defined under (d1) and (d2), PA1 R denotes a divalent group obtainable by removal of the isocyanate groups from a diisocyanate R(NCO).sub.2 of the kind defined above, PA1 R' denotes hydrogen or a monovalent hydrocarbon group with 1 to 8 carbon atoms, preferably hydrogen or a methyl group, PA1 R" denotes a monovalent hydrocarbon group with 1 to 12 carbon atoms, preferably an unsubstituted alkyl group with 1 to 4 carbon atoms, PA1 X denotes a group obtained by removal of the terminal oxygen atom from a polyalkylene oxide chain which has about 5 to 90, preferably about 20 to 70 chain members of which at least about 40%, preferably at least about 65% are ethylene oxide units, which chain may also contain propylene oxide, butylene oxide or styrene oxide units in addition to the ethylene oxide units, propylene oxide being preferred among these latter three mentioned oxides, PA1 Y denotes oxygen or NR'" in which R'" has the same meaning as R", and PA1 Z denotes --NH-- or a group conforming to the definition of Y.
This problem was solved by providing the polyurethanes according to the invention described below and the process for their preparation.